Thanks @jcl. That puts the size reduction is in the same ballpark, so I'll give the tiny trd a try.

FYI, I save a copy of the trd file most every weekend. I tested starting Zorro (in a test environment, not live) with these backups, and had to go back a number of weeks to find an un-corrupted trd. Zorro ran continuously over that period, and the weekly trd files have different sizes. Since Zorro was never stopped and restarted, the trd file was apparently never read - only written to. Unfortunately, especially given the passage of time, I have no hope of correlating the corruption to anything else. But from these circumstances a reasonable suspicion could be a rare pointer/buffer problem in Zorro?

HTH.